To achieve optical transmission systems at, for example, at least 40 Gbps, polarization-multiplexing is being studied (see Japanese Laid-open Patent Publication Nos. 2002-344426, 2003-338805, and 2005-65027, for example). In polarization-multiplexing, two mutually orthogonal polarized signals, for example, are multiplexed with the same wavelength and two independent signal information items are transmitted.
Polarization-multiplexing enables the use of a plurality of polarization states, so the baud rate of a signal to be transmitted is reduced and the frequency usage efficiency is increased. It is also known about polarization-multiplexing that a difference in power is caused between polarized optical signals because optical parts and optical transmission lines generates a polarization dependent loss (PDL), and transmission performance is thereby lowered (see O. Vassilieva et al. “Impact of Polarization Dependent Loss and Cross-Phase Modulation on Polarization Multiplexed DQPSK Signals”, OFC/NFOEC, 2008, for example).
In the prior art described above, however, the difference in power between polarized signals is difficult to measure. Therefore, for example, the difference in power between polarized signals cannot be precisely measured and optical transmission performance is thereby lowered.